Fuel cell power systems convert a fuel and an oxidant to electricity. One fuel cell power system type of keen interest employs use of a proton exchange membrane (hereinafter “PEM”) to catalytically facilitate reaction of fuels (such as hydrogen) and oxidants (such as air/oxygen) into electricity. The PEM is a solid polymer electrolyte that facilitates transfer of protons from the anode to the cathode in each individual fuel cell of the stack of fuel cells normally deployed in a fuel cell power system.
In a typical fuel cell assembly (stack) within a fuel cell power system, individual fuel cells have flow fields with inlets to fluid manifolds; these collectively provide channels for the various reactant and cooling fluids reacted in the stack to flow into each cell. Gas diffusion assemblies then provide a final fluid conduit to further distribute reactant fluids from the flow field space to the reactive anode and cathode.
Effective operation of a PEM requires a balanced provision of sufficient water in the polymer of a PEM to maintain its proton conductivity while maintaining the flow field channels and gas diffusion assemblies in non-flooded operational states. In this regard, the oxidant, typically oxygen or oxygen-containing air, is supplied to the cathode where it reacts with hydrogen cations that have crossed the proton exchange membrane and electrons from an external circuit. Thus, the fuel cell generates both electricity and water through the electrochemical reaction, and the water is removed with the cathode effluent, dehydrating the PEM of the fuel cell unless the water is otherwise replaced. It is also to be noted that the inlet air flow rate to the cathode will generally evaporate water from the proton exchange membrane at an even higher rate than the rate of water generation (and commensurate dehydration of the PEM) via reaction at the cathode.
When hydrated, the polymeric proton exchange membrane possesses “acidic” properties that provide a medium for conducting protons from the anode to the cathode of the fuel cell. However, if the proton exchange membrane is not sufficiently hydrated, the “acidic” character diminishes, with commensurate diminishment of the desired electrochemical reaction of the cell. Hydration of a fuel cell PEM also assists in temperature control within the fuel cell, insofar as the heat capacity of water provides a heat sink. Cooling of a fuel cell is assisted by the introduction of liquid water into the feed gases, especially when heat from the cell is used to provide the heat needed for evaporation.
In addition to issues in water balance and cell hydration, another issue in fuel cell design for use in vehicles is directed to the efficient use of space. In this regard, space in a vehicle is precious and approaches to design which minimize the permanent use of space in the vehicle clearly benefit the utility of the vehicle. This leads toward a desire to integrate the humidifying system into each of the fuel cells.
The need for efficiency in operation and for greater integration in cooling and humidification to achieve efficient space utilization in fuel cell systems continues to be strongly felt. Therefore, a need exists for a fuel cell power system which provides humidification of the feed gases (especially the oxidant), and in such a way that a minimum of space is needed for the humidification operation. The present invention is directed to fulfilling this need.